The bones of a skeletal system have a dense outer shell, or cortex, made of cortical bone (alternatively termed "compact" or "dense" bone). Inside the cortex, many types or regions of bone also have a mesh or network of trabecular bone (alternatively termed "spongy" or "cancellous" bone), made up of roughly the same material as the cortical bone. The region or regions enclosed by the cortex and/or the trabecular network contain marrow, and are aptly termed the "marrow space." In other types or regions of bone, particularly long bones such as femurs, the cortical bone encloses a trabecular-free marrow space, termed the "medulla" or the "medullary cavity." Regardless of whether it is contained in the trabecular network or the medullary cavity, marrow is comprised of living matter, including cells and a circulatory system, that supports the living constituents of the bone. Marrow also contains fat, and houses certain components of the blood-forming and immune systems of the body.
The dense bone material that makes up the cortical and trabecular bone is a composite material, made up of organic and inorganic constituents, intimately mixed. The organic constituents are alternately termed the "matrix" or "osteoid," and are comprised primarily of the protein collagen. The matrix is cartilage-like and flexible, and gives the bone material elasticity arid toughness.
The inorganic constituents are extremely small mineral crystals, carried and bound into a cohesive mass by the matrix, and give the bone its hardness and compressive strength. The crystals are typically flat plates on the order of hundreds of Angstroms or less on a side, and tens of Angstroms in thickness. The mineral crystals comprise about 60 to 70 percent of the total dry weight of dense bone.
A healthy human skeletal system among other things structurally supports the body, provides a set of levers for the mechanical actions of the skeletal muscles, protects the internal organs, houses parts of the blood-forming and immune systems, and functions as a storage reservoir for phosphate, calcium and other ions.
In an unhealthy skeletal system, one or more of these features may be compromised or lost. In the bone disease osteoporosis, the amount of bone in the skeleton is reduced, leading to weak and brittle bones and an increased risk of fracture. In Paget's disease, the rate at which bone mineral and matrix are cyclically resorbed (dissolved) and deposited is abnormally high, leading to distorted bone structure and pain. In osteomalacia (which is known as rickets when it affects growing children), there is an insufficient proportion of mineral to matrix. And in some instances, a bone fracture will not heal because of a failure in the bone reconstruction mechanism, a pathological condition known as nonunion.